Downing Street said Mr Cameron was being kept abreast of developments on Thursday, although it is understood he did not speak personally to either leader the day before the talks.

“Where is Britain?” said Sir Richard – until March the UK's most senior officer at Nato – in an interview with The Times.

“Where is Cameron? He is clearly a bit player. Nobody is taking any notice of him ... He is now a foreign policy irrelevance.”

On BBC Radio 4’s Today Programme, the general continued his criticism of Downing Street’s involvement and warned that failure to show strength will be counterproductive.

“Unless Nato speaks from a position of strength we are gifting the advantage potentially to Mr Putin. Wars start from a position of weakness, not as a result of strength. That dynamic once started gets worse if there is a sense of weakness in one side," Sir Richard said.

He added: “The UK is a major Nato member, it’s a major EU member, it’s a member of the UN security council, it’s a G8 member.

“It’s unfortunate that I think the weight the British PM could bring to efforts to resolve this crisis appear to be absent."